EP1166403A1 - Device for producing short pulses by passive mode lock - Google Patents

Device for producing short pulses by passive mode lock

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Publication number
EP1166403A1
EP1166403A1 EP00930991A EP00930991A EP1166403A1 EP 1166403 A1 EP1166403 A1 EP 1166403A1 EP 00930991 A EP00930991 A EP 00930991A EP 00930991 A EP00930991 A EP 00930991A EP 1166403 A1 EP1166403 A1 EP 1166403A1
Authority
EP
European Patent Office
Prior art keywords
laser
intensity
refractive index
reflector
diameter
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP00930991A
Other languages
German (de)
French (fr)
Other versions
EP1166403B1 (en
Inventor
Peter Glas
Martin Leitner
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Forschungsverbund Berlin FVB eV
Original Assignee
Forschungsverbund Berlin FVB eV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from DE19954109A external-priority patent/DE19954109C2/en
Application filed by Forschungsverbund Berlin FVB eV filed Critical Forschungsverbund Berlin FVB eV
Publication of EP1166403A1 publication Critical patent/EP1166403A1/en
Application granted granted Critical
Publication of EP1166403B1 publication Critical patent/EP1166403B1/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H01S3/1112—Passive mode locking
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3511—Self-focusing or self-trapping of light; Light-induced birefringence; Induced optical Kerr-effect
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3526—Non-linear optics using two-photon emission or absorption processes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08018—Mode suppression
    • H01S3/0804—Transverse or lateral modes
    • H01S3/0805—Transverse or lateral modes by apertures, e.g. pin-holes or knife-edges
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08059—Constructional details of the reflector, e.g. shape
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H01S3/1112—Passive mode locking
    • H01S3/1115—Passive mode locking using intracavity saturable absorbers
    • H01S3/1118—Semiconductor saturable absorbers, e.g. semiconductor saturable absorber mirrors [SESAMs]; Solid-state saturable absorbers, e.g. carbon nanotube [CNT] based

Definitions

  • the invention relates to a device for generating short pulses by passive mode coupling, in which an optical medium is arranged as a laser source between two reflectors and in which a reflector has optically non-linear properties.
  • EP 0805529 A2 describes a device for generating very short laser pulses, in which an optical medium is arranged as a laser source between two reflectors, in which a reflector is provided with semiconductor layers which absorb the incident laser beam as a function of intensity.
  • These components are designed as nonlinear reflectors in such a way that one or more semiconductor "quantum wells" (quantum walls) with a thickness of n- ⁇ / 2, where n is an odd number greater than 1, on a standard Bragg reflector which also consists of semiconductor material, are applied.
  • the semiconductor quantum wells must be produced at low temperatures so that the non-radiative recombination of the charge carriers generated by the radiation takes place in a very short time. By bleaching (saturation) the absorption of the special semiconductor material a loss modulation is generated which leads to the synchronization of the modes of the laser.
  • the known devices have the following disadvantages in particular. It is a very complex manufacturing technology
  • the manufacturing process which takes place at low temperatures, leads to strong tension in the material, which adversely affects both the linear optical and the spectroscopic properties (position of the absorption resonance).
  • the spectral position of the absorption resonance which must match the laser wavelength, can only be adjusted by a "trial and error” method, since both the composition of the semiconductor material (Ill-V connection) and the tension (growth temperature) are mutually related with respect to the influence the spectral position of the absorption resonance.
  • one side of the component can be provided with a dielectric mirror coating.
  • GaAs GaAs
  • interference effects in a 500 ⁇ m thick GaAs plate result in a modification of the refractive index, which is attributed to an influence on the pulse generation.
  • Rotation of the polarization plane leads, described as the mechanism of action for achieving mode synchronization.
  • a saturable absorber necessary to start the pulse operation.
  • a saturable absorber is used as an essential element for stimulating the pulse operation or for shaping the pulses in a cw-pumped laser.
  • the object of the invention is to provide a self-starting device for generating short pulses, which is simple to manufacture and does not require any adaptation of the laser wavelength to a spectrally narrow absorption resonance of the semiconductor material and can therefore be adapted to different lasers.
  • this object is achieved by a device of the type mentioned at the outset in that the optically non-linear reflector is formed from a refraction-controlled semiconductor layer which has an intensity-dependent refractive index and a highly reflective layer arranged behind it in the direction of the laser beam path.
  • the laser radiation is refracted differently in the refraction-controlled semiconductor layer depending on the respective intensity.
  • a radiation-induced charge carrier generation takes place through 2-photon absorption, - the energy of the laser photon hv based on the bandgap of the semiconductor material E g applies: hv ⁇ E g ⁇ 1, 4 hv, which is the charge carrier relaxation time much shorter than that Light circulation time between the two reflectors and greater than or equal to the duration of the pulses generated.
  • the refractive index change is intensity-dependent in the area of high intensity, there is a reduction in the
  • Refractive index in the low intensity range there is a
  • the linear absorption coefficient is not dependent on the intensity
  • the refraction-controlled material is sufficiently radiation-resistant for short pulses (destruction threshold> 10GW / cm 2 ).
  • a diverging lens is generated in this material by the laser radiation.
  • a number of materials basically meet these conditions, e.g. GaAs - ion-implanted (As, O) -, GaAs - grown at low temperatures (LT-GaAs) - and amorphous Si.
  • means for limiting the diameter of the laser beam and a lens system consisting of at least two lenses are arranged between the laser source and the non-linear reflector.
  • the first lens lying in the direction of the nonlinear reflector has a collimating effect and the second lens lying in the direction of the nonlinear reflector has a focusing effect.
  • the distance between the second lens and the refraction-controlled layer of the non-linear reflector is set in such a way that the diameter of the intensity distribution at the means for limiting the diameter of the laser beam for small intensities is greater than the diameter of the means for limiting the diameter of the laser beam.
  • the reflective layer of the nonlinear reflector, the reflectivity of which can be selected as a function of the materials used can be designed as a multilayer, for example consisting of dielectric materials or semiconductor materials, or as a metal coating.
  • short pulses can be generated in a self-starting manner in a laser.
  • the proposed arrangement is easier to produce than different semiconductor "quantum wells" from different materials, it can be integrated, does not require any adaptation of the laser wavelength to a spectrally narrow absorption resonance of the semiconductor material and can therefore be adapted to different lasers.
  • This device also compensates for refractive effects that occur due to non-linear interaction of the laser light with the materials present in the laser resonator, since the refractive index increments for the refraction-controlled semiconductor material and the optical media have opposite signs.
  • the compensation of the refractive effects can be set in a targeted manner.
  • Thickness that is greater than or equal to the wavelength used
  • Is laser radiation ie the layer thickness will be in the range of a few micrometers ( ⁇ m) depending on the wavelength. It is furthermore advantageous if the side of the refraction-controlled layer facing the laser source is non-reflective. Active temperature stabilization is not necessary.
  • a particular advantage of the proposed arrangement is that a strong non-linearity is realized over a very short light passage length.
  • Fig. 2 Representation of the diameter of the intensity distribution
  • a reinforcing medium serves as laser source 3, which is optically pumped by means of a cw diode laser or a diode laser array.
  • the diode radiation is coupled into the laser source 3 by known methods.
  • the laser source 3 can be in the form of a rod, a plate or a fiber.
  • the laser source 3 is located in a linear resonator, which is limited by the reflectors 1 and 2.
  • the reflector 1 is highly reflective for the laser wavelength, but highly transmissive for the pump radiation.
  • the resonator has a means for limiting the diameter of the laser beam in the beam path.
  • the laser source 3 consists of a rod-shaped or plate-shaped reinforcing medium
  • an aperture 4 is used as a means for limiting the diameter of the laser beam.
  • the laser source 3 consists of a fiber as a reinforcing medium
  • the fiber core takes over the function of the means for limiting the diameter of the laser beam.
  • the beam cross section of the laser radiation is suitably determined by the means for limiting the Diameter of the laser beam limited to the transverse basic mode.
  • the lens system with the two lenses 5 and 6 serves to collimate the laser beam delimited by the aperture 4 in the beam direction of the non-linear reflector 2 by means of the lens 5 and to focus on the non-linear reflector 2 by means of the lens 6.
  • the non-linear reflector 2 consists of the refraction-controlled layer 7 and of a highly reflective layer 8, which is arranged behind the refraction-controlled layer 7.
  • the non-linear reflector 2 is arranged on a support 9 for better handling.
  • a GaAs 2 photon absorber that was grown at low temperatures (low temperature [LT] grown GaAs) is used as the semiconductor material for the refraction-controlled layer 7.
  • the refraction-controlled layer 7 is formed with a thickness of a few micrometers ( ⁇ m).
  • the non-linear effect which is generated in the refraction-controlled layer 7 is proportional to the square of the intensity of the laser beam.
  • the reflectivity of the highly reflective layer 8 can be adjusted depending on the materials used. As a standard, it can be configured as a multilayer consisting of dielectric materials or semiconductor materials or as a metal coating.
  • the optically nonlinear reflector 2 is expediently arranged on a carrier 9, the task of which is to enable the adjustment of the nonlinear reflector 2 with respect to the incident laser radiation.
  • the front of the semiconductor material of the refraction-controlled layer 7 is advantageously antireflective.
  • the effect of the nonlinear effect in the refraction-controlled layer 7 is described below using the example of a semiconductor layer based on the laser beam geometry and the emission wavelength of the laser.
  • the free charge carriers generated by 2-photon absorption are determined by intensity, diffusion and recombination time.
  • the temporal change in the electron density is not determined by these two processes.
  • n e (ß / 2h ⁇ ) I 2 ⁇ , where I is the intensity h ⁇ the energy of the laser photon ß describes the 2-photon absorption, e.g. for ß:
  • GaAs, ion-implanted approx. 30 cm / GW LT-GaAs: 25- ⁇ ß ⁇ 45 cm / GW amorphous Si: approx. 52 cm / GW.
  • the change in intensity of the laser light in the direction of propagation z is
  • the Rayleigh length is large compared to the thickness d of the refraction-controlled layer 7, where the linear absorption coefficient is, for example, for ⁇ :
  • amorphous Si approx. 5 10 3 cm “ 1 .
  • I (2d) I 0 * (1 + Io ß d),
  • a charge carrier change is linked to a change in refractive index ⁇ n e according to the Drude model.
  • n ⁇ which is responsible for the magnitude of the refractive index change ⁇ n e
  • ⁇ n e ß C 1 0 2 ⁇ a / (2h ⁇ ) (1 + 1 0 ß2d-) 2 e.g. for ⁇ a :
  • GaAs, ion-implanted ⁇ 200 fs LT-GaAs: ⁇ 500 fs amorphous Si:> 800 fs.
  • the refractive index change which is caused by the Kerr effect and which has the same sign as the charge-related refractive index change, is:
  • condition for the energy of the laser photon h ⁇ in relation to the bandgap of the semiconductor material E g is: h ⁇ ⁇ E g ⁇ 1.4h ⁇ ,
  • the total phase change that the light experiences when passing through the refraction-controlled layer 7 is:
  • k 0 is the propagation constant for laser light.
  • the focal length f of the resulting negative lens ( ⁇ ⁇ 0, C ⁇ 0) is obtained taking into account the assumption that the refractive index distribution causes a quadratic phase shift and that the beam waist w is almost on the surface of the refraction-controlled layer 7.
  • the phase shift is in quadratic approximation:
  • the nonlinear effect that occurs in the optical materials penetrated by the laser radiation in the resonator, for example the medium of the laser source 3 and the lenses 5 and 6, is estimated as follows:
  • the refractive index change in the optical materials ⁇ n ⁇ err, Gias caused by the Kerr effect is:
  • ⁇ +1, 55-10 '16 cm 2 / W for optical materials (e.g. glass). Since ⁇ n Ke rt has a positive sign, optical materials show a focusing behavior depending on the intensity. As is known, the intensity-dependent change in the refractive index (phase) is not only responsible for self-defocusing (negative sign of ⁇ n) or self-focusing (positive sign of ⁇ n), but also for self-phase modulation.
  • the intensity-dependent phase change during the pulse course can be done according to the aforementioned relationships
  • the self-phase modulation (positive sign of ⁇ n) occurring in the laser material can be compensated for by self-phase modulation (negative sign of ⁇ n) in the semiconductor material of the refraction-controlled layer 7.
  • the refractive index change follows the pulse curve, i.e. that the charge carrier relaxation time is less than the pulse duration.
  • the material length ratio d ⁇ a i b i. / doia s can be in the range of 10 5 , for example then correspond to a layer thickness d Ha ibi. of 10 ⁇ m a glass path of 100 cm.
  • the refraction-controlled is in the semiconductor material Layer 7 generates a refractive index profile that reproduces itself spatially and temporally in accordance with the intensity distribution of the incident laser radiation.
  • the charge carriers generated in the semiconductor material of the refraction-controlled layer 7 by the striking laser radiation and the electronic Kerr effect make a contribution to the refractive index with a negative sign, i.e. the local intensity curve of the radiation corresponds to a refractive index curve in the semiconductor material of the refraction-controlled layer 7, with the points being of high intensity
  • the number of charge carriers generated is large, so the refractive index increment ⁇ n is also large in amount, but has a negative sign.
  • the refractive index distribution in the semiconductor material which occurs when irradiated with a laser beam which has a Gaussian intensity distribution has the effect that the optical path length d (n 0 - ⁇ n) is large in the edge regions of the distribution, the regions with low intensity, since the amount
  • the optical path length in the middle region of the distribution, the region with high intensity, is small because the amount
  • a diverging lens for the laser radiation is formed in the semiconductor material of the refraction-controlled layer 7. The focal length of this diverging lens depends on the shape of the intensity profile, the absolute intensity of the laser radiation and the size of the focal spot that is generated in the semiconductor material.
  • the semiconductor material of the refraction-controlled layer 7 is selected so that low absorption occurs for low-intensity laser radiation. Charge carriers are generated via 2-photon absorption.
  • the electronic Kerr effect is a property of the material under the influence of radiation.
  • the influence of the refraction-controlled by the intensity-dependent refractive index distribution in the semiconductor material Layer 7 produced a diverging lens on the radiation field distribution (diameter) described in the resonator. 2 only shows the beam path from the non-linear reflector 2 in the direction of the diaphragm 4.
  • the local intensity distribution for areas of low intensities in the temporal pulse profile has a larger diameter 12 than the local intensity distribution 11. which corresponds to the areas of the pulse with high intensity (pulse peak).
  • loss modulation is realized, which has a positive feedback characteristic (loss reduction) with increasing intensity.
  • the distance a of the lens 6 to the non-linear reflector 2 is selected such that the laser radiation with a high intensity in the reflected direction, from the non-linear reflector 2 in the direction of the first reflector 1, has a beam path 11 which has a smaller diameter at the diaphragm 4 than the aperture diameter, so that these reflected rays pass the aperture unhindered.
  • a beam path 12 is generated in the lens system, which has a larger diameter at the diaphragm 4 than the diaphragm diameter, so that these reflected rays cannot pass through the diaphragm.
  • the distance a between the lens 6 and the reflector 2 with the refraction-controlled layer 7 is determined by the non-linearity of the refraction-controlled layer 7, by the charge carrier relaxation time and by the laser laser 3, which is dependent on the pump power, and the pump power.
  • the magnitude of the diverging lens produced by the laser radiation in the refraction-controlled layer 7 can be determined from these variables in accordance with the aforementioned relationships. Taking this size into account the distance a between the lens 6 and the reflector 2 can be calculated using known dimensioning rules for the resonator.
  • the solid curve represents a section of the pulse shape, namely the right half pulse of the laser source 3 generated (laser beam), the intensity of the laser beam being plotted on the right vertical axis.
  • Curves A, B and C illustrate the change in transmission occurring at the aperture 4 or the fiber core of a fibrous laser source as a function of the distance a between the lens 6 and the refraction-controlled layer 7, the change in transmission being plotted on the left vertical axis .
  • the combination of the lens 6 and the diverging lens generated by the intensity circulating in the laser resonator must be designed in such a way that poorer transmission through the aperture or the fiber core is realized for small intensities compared to high intensities. In this way, an intensity-dependent (non-linear) loss modulation is realized, which leads to the coupling of the modes of the laser and thus to the generation of short pulses. 3 that the transmission of the laser beam at the aperture 4 decreases sharply with decreasing intensity. Due to the circulation of the pulse in the resonator, the radiation components with lower intensity go within a very short period of time lost at the aperture 4, so that only radiation components with high intensity remain in the resonator and thus a short pulse with high intensity is generated.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)
  • Lock And Its Accessories (AREA)
  • Semiconductor Lasers (AREA)

Abstract

Conventional devices for producing very short laser pulses in which an optical medium as the laser source is arranged between two reflectors are provided with a reflector that is provided with semiconductor layers that absorb the incident laser beam depending on the intensity thereof. The aim of the invention is to provide such a device that is easy to produce and does not require a complicated adjustment. To this end, the optical non-linear reflector (2) consists of a refraction-controlled semiconductor layer (7) that has an intensity-depending refractive index and a highly refractive layer (8). A diaphragm (4) and a lens system that consists of at least two lenses (5, 6) is arranged in the beam path between the laser source (3) and the non-linear reflector (2). The distance (a) between the lens (6) and the refraction-controlled layer (7) of the non-linear refractor (2) is adjusted in such a manner that the resulting diameter (12) of the intensity distribution is larger for small intensities than the diameter of the diaphragm (4). The inventive device is easy to produce, can be integrated and easily adapted to different lasers. The inventive device can be used to compensate refractive effects since the refractive index increments for the refraction-controlled semiconductor layer (7) and the optical media (3, 5, 6) have opposite algebraic signs.

Description

Bezeichnungdescription
Vorrichtung zur Erzeugung kurzer Impulse durch passive ModenkopplungDevice for generating short pulses by passive mode coupling
Beschreibungdescription
Die Erfindung betrifft eine Vorrichtung zur Erzeugung kurzer Impulse durch passive Modenkopplung, bei der ein optisches Medium als Laserquelle zwischen zwei Reflektoren angeordnet ist und bei der ein Reflektor optisch nichtlineare Eigenschaften aufweist.The invention relates to a device for generating short pulses by passive mode coupling, in which an optical medium is arranged as a laser source between two reflectors and in which a reflector has optically non-linear properties.
Die Entwicklung von Laserquellen mit anwenderspezifischen Eigenschaften ist die Aufgabe der modernen Lasertechnik. Für viele Anwendungen werden kompakte Laser, die im Impulsbetrieb hohe Leistungen generieren, benötigt. Üblicherweise werden kurze Impulse mittels Modensynchronisation erzeugt. Ein wesentlicher Fortschritt bei der Erzeugung kurzer Impulse wurde durch den Einsatz von Halbleiterbauelementen (z.B. Halbleiter-Quantenwall) erzielt. In der EP 0805529 A2 wird eine Vorrichtung zur Erzeugung sehr kurzer Laserimpulse beschrieben, bei der ein optisches Medium als Laserquelle zwischen zwei Reflektoren angeordnet ist, bei der ein Reflektor mit Halbleiterschichten versehen ist, die den auftreffenden Laserstrahl intensitätsabhängig absorbieren. Diese Bauelemente werden als nichtlineare Reflektoren derart ausgebildet, daß ein oder mehrere Halbleiter- "quantum wells" (Quantenwall) mit einer Dicke von n-λ/2, wobei n eine ungerade Zahl größer 1 ist, auf einen Standard-Bragg- Reflektor, der ebenfalls aus Halbleitermaterial besteht, aufgebracht werden. Die Herstellung der Halbleiter-"quantum wells" muß bei niedrigen Temperaturen erfolgen, damit die nichtstrahlende Rekombination der durch die Strahlung erzeugten Ladungsträger in sehr kurzer Zeit vor sich geht. Durch Ausbleichen (Sättigung) der Absorption des speziellen Halbleitermaterials wird eine Verlustmodulation erzeugt, die zur Synchronisation der Moden des Lasers führt.The development of laser sources with user-specific properties is the task of modern laser technology. For many applications, compact lasers that generate high power in pulse mode are required. Short pulses are usually generated using mode synchronization. Significant progress in the generation of short pulses has been achieved through the use of semiconductor components (eg semiconductor quantum wall). EP 0805529 A2 describes a device for generating very short laser pulses, in which an optical medium is arranged as a laser source between two reflectors, in which a reflector is provided with semiconductor layers which absorb the incident laser beam as a function of intensity. These components are designed as nonlinear reflectors in such a way that one or more semiconductor "quantum wells" (quantum walls) with a thickness of n-λ / 2, where n is an odd number greater than 1, on a standard Bragg reflector which also consists of semiconductor material, are applied. The semiconductor quantum wells must be produced at low temperatures so that the non-radiative recombination of the charge carriers generated by the radiation takes place in a very short time. By bleaching (saturation) the absorption of the special semiconductor material a loss modulation is generated which leads to the synchronization of the modes of the laser.
Den bekannten Vorrichtungen haften insbesondere folgende Nachteile an. Es ist eine sehr aufwendige HerstellungstechnologieThe known devices have the following disadvantages in particular. It is a very complex manufacturing technology
(Molekularstrahlepitaxie) erforderlich. Der Herstellungsprozeß, der bei tiefen Temperaturen abläuft, führt zu starken Verspannungen im Material, die sowohl die linearen optischen als auch die spektroskopischen Eigenschaften (Lage der Absorptionsresonanz) nachteilig beeinflussen. Die spektrale Lage der Absorptionsresonanz, die mit der Laserwellenlänge übereinstimmen muß, ist nur durch ein "trial and error"-Verfahren einstellbar, da sowohl die Zusammensetzung des Halbleitermaterials (Ill-V-Verbindung) als auch die Verspannung (Züchtungstemperatur) sich wechselseitig bezüglich der spektralen Lage der Absorptionsresonanz beeinflussen. Zur Anpassung der Laserintensität an die erforderliche Sättigungsintensität kann eine Seite des Bauelements (die dem Bragg-Reflektor abgewandte Seite) mit einer dielektrischen Verspiegelung versehen werden.(Molecular beam epitaxy) required. The manufacturing process, which takes place at low temperatures, leads to strong tension in the material, which adversely affects both the linear optical and the spectroscopic properties (position of the absorption resonance). The spectral position of the absorption resonance, which must match the laser wavelength, can only be adjusted by a "trial and error" method, since both the composition of the semiconductor material (Ill-V connection) and the tension (growth temperature) are mutually related with respect to the influence the spectral position of the absorption resonance. To adapt the laser intensity to the required saturation intensity, one side of the component (the side facing away from the Bragg reflector) can be provided with a dielectric mirror coating.
In "Gallium arsenide: A new material to accomplish passively mode-locked Nd:YAG laser" ZHANG, Z; u. a. -Appl. Phys. Lett.Nol. 60, Νo. 4, 1992, S. 419 - 421 wird die Erzeugung kurzer Impulse durch passive Modenkopplung in einem YAG Laser unter Zuhilfenahme von semi-insulating GaAs beschrieben. Die Impulserzeugung wird auf die Wirkung von sättigbarer Absorption im Ga As zurückgeführt. Es wird ebenfalls darauf hingewiesen, daß durch Interferenzeffekte in einer 500μm dicken GaAs Platte eine Modifizierung des Brechungsindex auftritt, der ein Einfluß auf die Impulserzeugung zugeschrieben wird.In "Gallium arsenide: A new material to accomplish passively mode-locked Nd: YAG laser" ZHANG, Z; u. a. -Appl. Phys. Lett.Nol. 60, Νo. 4, 1992, pp. 419 - 421 describes the generation of short pulses by passive mode coupling in a YAG laser with the aid of semi-insulating GaAs. The pulse generation is attributed to the effect of saturable absorption in the Ga As. It is also pointed out that interference effects in a 500 μm thick GaAs plate result in a modification of the refractive index, which is attributed to an influence on the pulse generation.
In der DE 196 42 925 A1 wird der Kerr Effekt, der im Glas über einen nichtlinearen Prozeß zu einer Einschnürung des Lichtbündels und zurIn DE 196 42 925 A1 the Kerr effect, which constricts the light beam in the glass via a non-linear process and
Drehung der Polarisationsebene führt, als Wirkmechanismus zur Erzielung von Modensynchronisation beschrieben. Zusätzlich ist in einer solchen Anordnung noch ein sättigbarer Absorber notwendig, um den Impulsbetrieb zu starten.Rotation of the polarization plane leads, described as the mechanism of action for achieving mode synchronization. In addition is in one Arrangement still a saturable absorber necessary to start the pulse operation.
Eine analoge Lösung wird von Keller, U. "Ultrafast all-solid State laser technology" in Appl. Phys. B, Vol. 58,1994, S. 347 - 363 beschrieben. Als wesentliches Element zum Anfachen des Impulsbetriebs oder zur Formung der Impulse in einem cw-gepumpten Laser wird ein sättigbarer Absorber verwendet.An analog solution is provided by Keller, U. "Ultrafast all-solid state laser technology" in Appl. Phys. B, Vol. 58, 1994, pp. 347-363. A saturable absorber is used as an essential element for stimulating the pulse operation or for shaping the pulses in a cw-pumped laser.
Die Verwendung dieser nichtlinearen Reflektoren bietet den Vorteil, daß der Prozeß der Impulserzeugung selbststartend ist.The use of these non-linear reflectors offers the advantage that the pulse generation process is self-starting.
Die Aufgabe der Erfindung besteht darin, eine selbststartende Vorrichtung zur Erzeugung kurzer Impulse anzugeben, die sich einfach herstellen läßt und keine Anpassung der Laserwellenlänge an eine spektral schmale Absorptionsresonanz des Halbleitermaterials erfordert und somit an unterschiedliche Laser anpaßbar ist.The object of the invention is to provide a self-starting device for generating short pulses, which is simple to manufacture and does not require any adaptation of the laser wavelength to a spectrally narrow absorption resonance of the semiconductor material and can therefore be adapted to different lasers.
Erfindungsgemäß wird diese Aufgabe durch eine Vorrichtung der eingangs genannten Art dadurch gelöst, daß der optisch nichtlineare Reflektor aus einer refraktionsgesteuerten Halbleiterschicht, die einen intensitätsabhängigen Brechungsindex aufweist und einer in Richtung des Laserstrahlverlaufs dahinter angeordneten hochreflektierenden Schicht gebildet ist. In der refraktionsgesteuerten Halbleiterschicht wird die Laserstrahlung in Abhängigkeit von der jeweiligen Intensität unterschiedlich gebrochen. Für das Halbleitermaterial der refraktionsgesteuerten Halbleiterschicht gelten folgende Bedingungen: eine strahlungsinduzierte Ladungsträgergeneration findet durch 2- Photonenabsorption statt, - für die Energie des Laserphotons hv bezogen auf den Bandabstand des Halbleitermaterials Eg gilt: hv < Eg < 1 ,4 hv, die Ladungsträgerrelaxationszeit ist wesentlich kürzer als die Lichtumlaufzeit zwischen den beiden Reflektoren und größer oder gleich der Dauer der erzeugten Impulse. die Brechungsindexänderung ist intensitätsabhängig im Bereich großer Intensität kommt es zu einer Verringerung desAccording to the invention, this object is achieved by a device of the type mentioned at the outset in that the optically non-linear reflector is formed from a refraction-controlled semiconductor layer which has an intensity-dependent refractive index and a highly reflective layer arranged behind it in the direction of the laser beam path. The laser radiation is refracted differently in the refraction-controlled semiconductor layer depending on the respective intensity. The following conditions apply to the semiconductor material of the refraction-controlled semiconductor layer: a radiation-induced charge carrier generation takes place through 2-photon absorption, - the energy of the laser photon hv based on the bandgap of the semiconductor material E g applies: hv <E g <1, 4 hv, which is the charge carrier relaxation time much shorter than that Light circulation time between the two reflectors and greater than or equal to the duration of the pulses generated. the refractive index change is intensity-dependent in the area of high intensity, there is a reduction in the
Brechungsindex, im Bereich kleiner Intensität kommt es zu einerRefractive index, in the low intensity range there is a
Vergrößerung des Brechungsindex, die intensitätsabhängige Änderung des Brechungsindex ist groß imEnlargement of the refractive index, the intensity-dependent change in the refractive index is large in
Vergleich zu allen anderen Brechungsindexänderungen der lineare Absorptionskoeffizient ist nicht von der Intensität abhängig, das refraktionsgesteuerte Material ist für kurze Impulse hinreichend strahlungsfest (Zerstörschwelle >10GW / cm2).Compared to all other refractive index changes, the linear absorption coefficient is not dependent on the intensity, the refraction-controlled material is sufficiently radiation-resistant for short pulses (destruction threshold> 10GW / cm 2 ).
In diesem Material wird durch die Laserstrahlung eine Zerstreuungslinse erzeugt.A diverging lens is generated in this material by the laser radiation.
Eine Reihe von Materialien erfüllt grundsätzlich diese Bedingungen, z.B. GaAs - ionenimplantiert (As, O)-, GaAs -bei niedrigen Temperaturen gewachsen (LT-GaAs)- und amorphes Si.A number of materials basically meet these conditions, e.g. GaAs - ion-implanted (As, O) -, GaAs - grown at low temperatures (LT-GaAs) - and amorphous Si.
Im Strahlenverlauf sind zwischen der Laserquelle und dem nichtlinearen Reflektor Mittel zur Begrenzung des Durchmessers des Laserstrahls und ein aus mindestens zwei Linsen bestehendes Linsensystem angeordnet. Die in Richtung auf den nichtlinearen Reflektor liegende erste Linse hat eine kollimierende Wirkung und die in Richtung auf den nichtlinearen Reflektor liegende zweite Linse hat eine fokussierende Wirkung. Der Abstand zwischen der zweiten Linse und der refraktionsgesteuerten Schicht des nichtlinearen Reflektors ist derart eingestellt, daß der sich einstellende Durchmesser der Intensitätsverteilung an dem Mittel zur Begrenzung des Durchmessers des Laserstrahls für kleine Intensitäten größer ist als der Durchmesser des Mittels zur Begrenzung des Durchmessers des Laserstrahls. Die reflektierende Schicht des nichtlinearen Reflektors, deren Reflexionsvermögen in Abhängigkeit der verwendeten Materialien wählbar ist, kann als Vielfachschicht, beispielsweise bestehend aus dielektrischen Materialien oder Halbleitermaterialien oder als Metallbelegung ausgebildet sein.In the beam path, means for limiting the diameter of the laser beam and a lens system consisting of at least two lenses are arranged between the laser source and the non-linear reflector. The first lens lying in the direction of the nonlinear reflector has a collimating effect and the second lens lying in the direction of the nonlinear reflector has a focusing effect. The distance between the second lens and the refraction-controlled layer of the non-linear reflector is set in such a way that the diameter of the intensity distribution at the means for limiting the diameter of the laser beam for small intensities is greater than the diameter of the means for limiting the diameter of the laser beam. The reflective layer of the nonlinear reflector, the reflectivity of which can be selected as a function of the materials used, can be designed as a multilayer, for example consisting of dielectric materials or semiconductor materials, or as a metal coating.
Mit der erfindungsgemäßen Vorrichtung lassen sich in einem Laser selbststartend kurze Impulse erzeugen. Die vorgeschlagene Anordnung ist im Vergleich zur Realisierung von Halbleiter-"quantum wells" aus verschiedenen Materialien einfacher herzustellen, sie ist integrierbar, erfordert keine Anpassung der Laserwellenlänge an eine spektral schmale Absorptionsresonanz des Halbleitermaterials und ist daher an unterschiedliche Laser anpaßbar.With the device according to the invention, short pulses can be generated in a self-starting manner in a laser. The proposed arrangement is easier to produce than different semiconductor "quantum wells" from different materials, it can be integrated, does not require any adaptation of the laser wavelength to a spectrally narrow absorption resonance of the semiconductor material and can therefore be adapted to different lasers.
Mit dieser Vorrichtung erfolgt auch eine Kompensation refraktiver Effekte, die durch nichtlineare Wechselwirkung des Laserlichts mit den im Laserresonator vorhandenen Materialien auftreten, da die Brechungsindexinkremente für das refraktionsgesteuerte Halbleitermaterial und die optischen Medien umgekehrte Vorzeichen aufweisen. Durch geeignete Dimensionierung der Schichtdicke der refraktionsgesteuerten Halbleiterschicht sowie der Wege in optischen Materialien (z. B. Glas) kann die Kompensation der refraktiven Effekte gezielt eingestellt werden.This device also compensates for refractive effects that occur due to non-linear interaction of the laser light with the materials present in the laser resonator, since the refractive index increments for the refraction-controlled semiconductor material and the optical media have opposite signs. By suitable dimensioning of the layer thickness of the refraction-controlled semiconductor layer and the paths in optical materials (e.g. glass), the compensation of the refractive effects can be set in a targeted manner.
Weitere vorteilhafte Ausgestaltungen der Vorrichtung können den Unteransprüchen entnommen werden. So ist es beispielsweise vorteilhaft, den optisch nichtlinearen Reflektor auf einem Träger anzuordnen.Further advantageous embodiments of the device can be found in the subclaims. For example, it is advantageous to arrange the optically nonlinear reflector on a support.
Die refraktionsgesteuerte Schicht besitzt vorteilhafterweise eine optischeThe refraction-controlled layer advantageously has an optical one
Dicke, die größer oder gleich der verwendeten Wellenlänge derThickness that is greater than or equal to the wavelength used
Laserstrahlung ist, d.h. die Schichtdicke wird je nach Wellenlänge im Bereich einiger Mikrometer (μm) liegen. Weiterhin ist es vorteilhaft, wenn die der Laserquelle zugewandte Seite der refraktionsgesteuerten Schicht entspiegelt ist. Eine aktive Temperaturstabilisierung ist nicht erforderlich.Is laser radiation, ie the layer thickness will be in the range of a few micrometers (μm) depending on the wavelength. It is furthermore advantageous if the side of the refraction-controlled layer facing the laser source is non-reflective. Active temperature stabilization is not necessary.
Ein besonderer Vorteil der vorgeschlagene Anordnung besteht darin, daß eine starke Nichtlinearität auf einer sehr kurzen Lichtdurchlauflänge realisiert wird.A particular advantage of the proposed arrangement is that a strong non-linearity is realized over a very short light passage length.
Die Erfindung soll anhand eines Ausführungsbeispiels näher erläutert werden.The invention will be explained in more detail using an exemplary embodiment.
Die dazugehörigen Zeichnungen zeigen:The associated drawings show:
Fig. 1 : Vorrichtung zur Erzeugung kurzer Impulse,1: device for generating short pulses,
Fig. 2: Darstellung der Durchmesser der IntensitätsverteilungFig. 2: Representation of the diameter of the intensity distribution
Fig. 3: Darstellung der VerlustmodulationFig. 3: Representation of loss modulation
Eine Ausführungsform der Vorrichtung zur Erzeugung kurzer Impulse ist in Fig. 1 dargestellt. Ein verstärkendes Medium dient als Laserquelle 3, die mittels eines cw-Diodenlasers oder eines Diodenlaser-Arrays optisch gepumpt wird. Die Einkopplung der Diodenstrahlung in die Laserquelle 3 erfolgt nach bekannten Methoden. Die Laserquelle 3 kann in Form eines Stabs, einer Platte oder einer Faser ausgebildet sein. Die Laserquelle 3 befindet sich in einem linearen Resonator, der durch die Reflektoren 1 und 2 begrenzt wird. Der Reflektor 1 ist für die Laserwellenlänge hoch reflektierend, für die Pumpstrahlung jedoch hoch transmittierend. Der Resonator weist im Strahlenverlauf ein Mittel zur Begrenzung des Durchmessers des Laserstrahls auf. Besteht die Laserquelle 3 aus einem stab- bzw. plattenförmigen verstärkenden Medium wird als Mittel zur Begrenzung des Durchmessers des Laserstrahls eine Blende 4 verwendet. Besteht die Laserquelle 3 aus einer Faser als verstärkendes Medium, übernimmt der Faserkern die Funktion des Mittels zur Begrenzung des Durchmessers des Laserstrahls. Der Strahlquerschnitt der Laserstrahlung wird in geeigneter Weise durch das Mittel zur Begrenzung des Durchmessers des Laserstrahls auf den transversalen Grundmode beschränkt. Das Linsensystem mit den beiden Linsen 5 und 6 dient dazu, den durch die Blende 4 begrenzten Laserstrahl in Strahlrichtung des nichtlinearen Reflektors 2 mittels der Linse 5 zu kollimieren und mittels der Linse 6 auf den nichtlinearen Reflektor 2 zu fokussieren. Der nichtlineare Reflektor 2 besteht aus der refraktionsgesteuerten Schicht 7 und aus einer hochreflektierenden Schicht 8, die hinter der refraktionsgesteuerten Schicht 7 angeordnet ist. Der nichtlineare Reflektor 2 ist zur besseren Handhabung auf einem Träger 9 angeordnet.An embodiment of the device for generating short pulses is shown in FIG. 1. A reinforcing medium serves as laser source 3, which is optically pumped by means of a cw diode laser or a diode laser array. The diode radiation is coupled into the laser source 3 by known methods. The laser source 3 can be in the form of a rod, a plate or a fiber. The laser source 3 is located in a linear resonator, which is limited by the reflectors 1 and 2. The reflector 1 is highly reflective for the laser wavelength, but highly transmissive for the pump radiation. The resonator has a means for limiting the diameter of the laser beam in the beam path. If the laser source 3 consists of a rod-shaped or plate-shaped reinforcing medium, an aperture 4 is used as a means for limiting the diameter of the laser beam. If the laser source 3 consists of a fiber as a reinforcing medium, the fiber core takes over the function of the means for limiting the diameter of the laser beam. The beam cross section of the laser radiation is suitably determined by the means for limiting the Diameter of the laser beam limited to the transverse basic mode. The lens system with the two lenses 5 and 6 serves to collimate the laser beam delimited by the aperture 4 in the beam direction of the non-linear reflector 2 by means of the lens 5 and to focus on the non-linear reflector 2 by means of the lens 6. The non-linear reflector 2 consists of the refraction-controlled layer 7 and of a highly reflective layer 8, which is arranged behind the refraction-controlled layer 7. The non-linear reflector 2 is arranged on a support 9 for better handling.
In diesem Beispiel wird als Halbleitermaterial für die refraktionsgesteuerte Schicht 7 ein GaAs 2-Photonenabsorber, der bei niedrigen Temperaturen gezüchtet wurde (low temperature [LT] grown GaAs) verwendet. Die refraktionsgesteuerte Schicht 7 ist mit einer Dicke von einigen Mikrometern (μm) ausgebildet. Der nichtlineare Effekt, der in der refraktionsgesteuerten Schicht 7 erzeugt wird, ist proportional zum Quadrat der Intensität des Laserstrahls. Das Reflexionsvermögen der hochreflektierenden Schicht 8 kann in Abhängigkeit von den verwendeten Materialien eingestellt werden. Sie kann standardmäßig als Vielfachschicht, bestehend aus dielektrischen Materialien oder Halbleitermaterialien oder als Metallbelegung ausgebildet sein.In this example, a GaAs 2 photon absorber that was grown at low temperatures (low temperature [LT] grown GaAs) is used as the semiconductor material for the refraction-controlled layer 7. The refraction-controlled layer 7 is formed with a thickness of a few micrometers (μm). The non-linear effect which is generated in the refraction-controlled layer 7 is proportional to the square of the intensity of the laser beam. The reflectivity of the highly reflective layer 8 can be adjusted depending on the materials used. As a standard, it can be configured as a multilayer consisting of dielectric materials or semiconductor materials or as a metal coating.
Zweckmäßigerweise ist der optisch nichtlineare Reflektor 2 auf einem Träger 9 angeordnet, dessen Aufgabe darin besteht, die Justierung des nichtlinearen Reflektors 2 hinsichtlich der auftreffenden Laserstrahlung zu ermöglichen.The optically nonlinear reflector 2 is expediently arranged on a carrier 9, the task of which is to enable the adjustment of the nonlinear reflector 2 with respect to the incident laser radiation.
Vorteilhafterweise wird zur Vermeidung unerwünschter Fabry-Perot-Effekte die Vorderseite des Halbleitermaterials der refraktionsgesteuerten Schicht 7 entspiegelt. Die Wirkung des nichtlinearen Effekts in der refraktionsgesteuerten Schicht 7 wird am Beispiel einer Halbleiterschicht bezogen auf die Laserstrahlgeometrie und die Emissionswellenlänge des Lasers nachfolgend beschrieben.In order to avoid undesirable Fabry-Perot effects, the front of the semiconductor material of the refraction-controlled layer 7 is advantageously antireflective. The effect of the nonlinear effect in the refraction-controlled layer 7 is described below using the example of a semiconductor layer based on the laser beam geometry and the emission wavelength of the laser.
Die durch 2-Photonenabsorption generierten freien Ladungsträger sind durch Intensität, Diffusions- und Rekombinationszeit bestimmt. Für Impulse, deren Dauer τ klein im Vergleich zur Diffusions- und Rekombinationszeit ist, ist die zeitliche Änderung der Elektronendichte nicht durch diese beiden Prozesse bestimmt. Man erhält für die zeitliche Änderung der Ladungsträgerkonzentration in Abhängigkeit der IntensitätsverteilungThe free charge carriers generated by 2-photon absorption are determined by intensity, diffusion and recombination time. For impulses whose duration τ is small in comparison to the diffusion and recombination time, the temporal change in the electron density is not determined by these two processes. One obtains for the change in the charge carrier concentration as a function of the intensity distribution
dne/dt = (ß / 2hυ) f (t),dn e / dt = (ß / 2hυ) f (t),
daraus ergibt sich für die Ladungsträgerkonzentrationthis results in the charge carrier concentration
ne = (ß / 2hυ) I2τ , dabei ist I die Intensität hυ die Energie des Laserphotons ß beschreibt die 2-Photonenabsorption z.B. gilt für ß:n e = (ß / 2hυ) I 2 τ, where I is the intensity hυ the energy of the laser photon ß describes the 2-photon absorption, e.g. for ß:
GaAs, ionenimplantiert: ca. 30 cm/GW LT-GaAs: 25-< ß < 45 cm/GW amorphes Si: ca. 52 cm/GW. Die Intensitätsänderung des Laserlichts in der Ausbreitungsrichtung z ist,GaAs, ion-implanted: approx. 30 cm / GW LT-GaAs: 25- <ß <45 cm / GW amorphous Si: approx. 52 cm / GW. The change in intensity of the laser light in the direction of propagation z is
dl(z,t)/dz = - α l(z,t)-ß l 2(z,t),dl (z, t) / dz = - α l (z, t) -ß l 2 (z, t),
vorausgesetzt die Rayleighlänge ist groß im Vergleich zur Dicke d der refraktionsgesteuerten Schicht 7, dabei ist der lineare Absorptionskoeffizient z.B. gilt für α:provided the Rayleigh length is large compared to the thickness d of the refraction-controlled layer 7, where the linear absorption coefficient is, for example, for α:
GaAs, ionenimplantiert: ca. 5 103 cm" LT-GaAs: 100 cm"1 < α < 1000 cm"1 amorphes Si: ca. 5 103 cm"1.GaAs, ion-implanted: approx. 5 10 3 cm " LT-GaAs: 100 cm " 1 <α <1000 cm "1 amorphous Si: approx. 5 10 3 cm " 1 .
Wird die lineare Absorption vernachlässigt, erhält man für die Intensität nach zweimaligem Durchgang durch die refraktionsgesteuerte Schicht 7 mit der Dicke dIf the linear absorption is neglected, the intensity is obtained after two passes through the refraction-controlled layer 7 with the thickness d
I(2d) = I 0 * (1 + Io ß d),I (2d) = I 0 * (1 + Io ß d),
vorausgesetzt wird dabei, daß der Reflexionsgrad der hochreflektierenden Schicht 8, die sich hinter der refraktionsgesteuerten Schicht 7 befindet, nahezu 100% beträgt.it is assumed that the reflectance of the highly reflective layer 8, which is located behind the refraction-controlled layer 7, is almost 100%.
Eine Ladungsträgeränderung ist nach dem Drude-Modell mit einer Brechungsindexänderung Δne verknüpft.A charge carrier change is linked to a change in refractive index Δn e according to the Drude model.
Diese beträgt Δnθ = C ne, dabei ist C = - e2 p / (2-n0 ε0 mθh ω2) n0 ist der Brechungsindex des Mediums, ε0 ist die Dielektrizitätskonstante des Mediums, ιτieh ist die reduzierte Masse des Elektron-Loch- Paars, ω ist die zirkuläre Laserfrequenz, p berücksichtigt Beiträge zum nichtlinearen Brechungsindex, die nicht durch das Drude- Modell beschrieben werden. Um die Ladungsträgerkonzentration nθ, die für die Größe der Brechungsindexänderung Δne verantwortlich ist, zu bestimmen, integriert man dne/dt über die Ladungsträgerrelaxationszeit τa. Unter Verwendung der Intensität I(2d) erhält man für Δne:This is Δn θ = C n e , where C = - e 2 p / (2-n 0 ε 0 m θh ω 2 ) n 0 is the refractive index of the medium, ε 0 is the dielectric constant of the medium, ιτi eh is that reduced mass of the electron-hole pair, ω is the circular laser frequency, p takes into account contributions to the nonlinear refractive index that are not described by the Drude model. In order to determine the charge carrier concentration n θ , which is responsible for the magnitude of the refractive index change Δn e , one integrates dn e / dt over the charge carrier relaxation time τ a . Using intensity I (2d) one obtains for Δn e :
Δne = ß C 10 2 τ a / (2hυ) (1 + 10ß2d-)2 z.B. gilt für τa:Δn e = ß C 1 0 2 τ a / (2hυ) (1 + 1 0 ß2d-) 2 e.g. for τ a :
GaAs, ionenimplantiert: < 200 fs LT-GaAs: < 500 fs amorphes Si: > 800 fs.GaAs, ion-implanted: <200 fs LT-GaAs: <500 fs amorphous Si:> 800 fs.
Dieser Ausdruck ist als Näherung zu betrachten, da die oben gemachten Einschränkungen bezüglich der Impulsdauer zu berücksichtigen sind und weiterhin eine Änderung der Impulsintensität während der Impulsdauer nicht berücksichtigt wird.This expression is to be regarded as an approximation since the restrictions made regarding the pulse duration have to be taken into account and furthermore a change in the pulse intensity during the pulse duration is not taken into account.
Die Brechungsindexänderung, die durch den Kerr-Effekt hervorgerufen wird und die das gleiche Vorzeichen wie die ladungsträgerbedingte Brechungsindexänderung besitzt, ist:The refractive index change, which is caused by the Kerr effect and which has the same sign as the charge-related refractive index change, is:
ΔnKerr = J IΔn K err = JI
unter der Voraussetzung, daß für die Energie des Laserphotons hυ bezogen auf den Bandabstand des Halbleitermateriais Eg die Bedingung gilt: hυ < Eg < 1.4hυ,provided that the condition for the energy of the laser photon hυ in relation to the bandgap of the semiconductor material E g is: hυ <E g <1.4hυ,
wobei der Koeffizient γ mittels folgender Gleichung aus dem nichtlinearen Brechungsindex n2 ermittelt wird: γ = 4 π n27 n z.B. gilt für γ.where the coefficient γ is determined from the nonlinear refractive index n 2 using the following equation: γ = 4 π n 2 7 n, for example, applies to γ.
GaAs, ionenimplantiert: -3,2-10'13 cm2 / W LT-GaAs: -3,5-10"13 cm2 / W amorphes Si: (muß experimentell ermittelt werden)GaAs, ion-implanted: -3.2-10 '13 cm 2 / W LT-GaAs: -3.5-10 "13 cm 2 / W amorphous Si: (must be determined experimentally)
Die totale Phasenänderung, die das Licht beim Durchgang durch die refraktionsgesteuerte Schicht 7 erfährt, beträgt:The total phase change that the light experiences when passing through the refraction-controlled layer 7 is:
ΔΦ = 2d k0 (Δne + ΔnKerr)ΔΦ = 2d k 0 (Δn e + Δn Ke rr)
= 2d k0 (γ-I + ß-C τa I0 2 / (2hυ) (1+2 d Io-ß)2),= 2d k 0 (γ-I + ß-C τ a I 0 2 / (2hυ) (1 + 2 d Io-ß) 2 ),
dabei ist k0 die Ausbreitungskonstante für Laserlicht.where k 0 is the propagation constant for laser light.
Die Brennweite f der resultierenden negativen Linse (γ < 0, C<0) erhält man unter Berücksichtigung der Annahme, daß die Brechungsindexverteilung eine quadratische Phasenverschiebung bewirkt und daß die Strahltaille w nahezu auf der Oberfläche der refraktionsgesteuerten Schicht 7 liegt. Die Phasenverschiebung ist in quadratischer Näherung:The focal length f of the resulting negative lens (γ <0, C <0) is obtained taking into account the assumption that the refractive index distribution causes a quadratic phase shift and that the beam waist w is almost on the surface of the refraction-controlled layer 7. The phase shift is in quadratic approximation:
ΔΦ = k0w2/ 2 f,ΔΦ = k 0 w 2/2 f,
woraus sich die Brennweite f ermitteln läßt:from which the focal length f can be determined:
f = k0 w2 / 2 ΔΦ = w2 / 2 d (γl + ß C τa I0 2 / (2 hυ) (1+2 d Io ß)2).f = k 0 w 2/2 ΔΦ = w 2/2 d (γl + ß C τ a I 0 2 / (2 hυ) (1 + 2 d I o ß) 2 ).
Der nichtlineare Effekt, der in den von der Laserstrahlung durchsetzten optischen Materialien im Resonator, beispielsweise dem Medium der Laserquelle 3 sowie der Linsen 5 und 6, auftritt, wird wie folgt abgeschätzt:The nonlinear effect that occurs in the optical materials penetrated by the laser radiation in the resonator, for example the medium of the laser source 3 and the lenses 5 and 6, is estimated as follows:
Die durch den Kerr-Effekt hervorgerufene Brechungsindexänderung in den optischen Materialien Δnκerr,Gias beträgt:The refractive index change in the optical materials Δnκerr, Gias caused by the Kerr effect is:
ΔnKer ,Glas =Δn K er, glass =
wobei γ = +1 ,55-10'16cm2 / W für optische Materialien (z.B. Glas) ist. Da ΔnKert ein positives Vorzeichen besitzt, zeigen optische Materialien in Abhängigkeit von der Intensität ein fokussierendes Verhalten. Bekanntlich ist die intensitätsabhängige Brechungsindex-(Phasen)-änderung nicht nur für Selbstdefokussierung (negatives Vorzeichen von Δn) oder Selbstfokussierung (positives Vorzeichen von Δn) verantwortlich sondern auch für Selbstphasenmodulation. Die intensitätsabhängige Phasenänderung während des Impulsverlaufs läßt sich nach vorgenannten Beziehungen mitwhere γ = +1, 55-10 '16 cm 2 / W for optical materials (e.g. glass). Since Δn Ke rt has a positive sign, optical materials show a focusing behavior depending on the intensity. As is known, the intensity-dependent change in the refractive index (phase) is not only responsible for self-defocusing (negative sign of Δn) or self-focusing (positive sign of Δn), but also for self-phase modulation. The intensity-dependent phase change during the pulse course can be done according to the aforementioned relationships
ΔΦ = 2-d k0-(Δne+ΔnKett,Haibi) bzw. ΔΦG|aS = 2-d-k0-ΔnKerr,GlasΔΦ = 2-dk 0 - (An e + K .DELTA.n ett, Haibi) or G ΔΦ | aS = 2-dk 0 -Δn K err, glass
angeben, d.h., daß die im Lasermaterial auftretende Selbstphasenmodulation (positives Vorzeichen von Δn) durch eine Selbstphasenmodulation (negatives Vorzeichen von Δn) im Halbleitermaterial der refraktionsgesteuerten Schicht 7 kompensiert werden kann. Voraussetzung dafür ist, daß die Brechungsindexänderung dem Impulsverlauf folgt d.h., daß die Ladungsträgerrelaxationszeit kleiner als die Impulsdauer ist. Dabei kann der Betrag des Verhältnissesindicate, i.e. that the self-phase modulation (positive sign of Δn) occurring in the laser material can be compensated for by self-phase modulation (negative sign of Δn) in the semiconductor material of the refraction-controlled layer 7. The prerequisite for this is that the refractive index change follows the pulse curve, i.e. that the charge carrier relaxation time is less than the pulse duration. The amount of the ratio
(Δne+ΔnKerr,Haibi) / ΔnKerr,Gias) I ~ 105 betragen.(Δn e + Δn Ke rr, Haibi) / Δn Ke rr, Gias) I ~ 10 5 .
Daraus folgt, daß eine Kompensation des nichtlinearen Effekts, der in den von der Laserstrahlung durchsetzten optischen Materialien im Resonator auftritt, durch den nichtlinearen Effekt im Halbleitermaterial im vorgenannten Verhältnis kompensierbar ist. Das bedeutet, daß für eine Kompensation von Phasenänderungen das Materiallängenverhältnis dπaibi. / doias im Bereich von 105 liegen kann, z.B. entsprechen dann einer Schichtdicke dHaibi. von 10 μm ein Glasweg von 100 cm.It follows from this that a compensation of the nonlinear effect which occurs in the optical materials in the resonator penetrated by the laser radiation can be compensated for by the nonlinear effect in the semiconductor material in the aforementioned ratio. This means that for a compensation of phase changes, the material length ratio dπ a i b i. / doia s can be in the range of 10 5 , for example then correspond to a layer thickness d Ha ibi. of 10 μm a glass path of 100 cm.
Unter Kenntnis und gezielter Ausnutzung der vorgenannten physikalischen Zusammenhänge wird in dem Halbleitermaterial der refraktionsgesteuerten Schicht 7 ein Brechungsindexprofil erzeugt, das sich räumlich und zeitlich der Intensitätsverteilung der auffallenden Laserstrahlung entsprechend nachbildet. Die im Halbleitermaterial der refraktionsgesteuerten Schicht 7 durch die auffallende Laserstrahlung erzeugten Ladungsträger und der elektronische Kerr-Effekt ergeben einen Beitrag zum Brechungsindex mit negativem Vorzeichen, d.h. der örtliche Intensitätsverlauf der Strahlung entspricht einem Brechungsindexverlauf im Halbleitermaterial der refraktionsgesteuerten Schicht 7, wobei an Stellen hoher Intensität die Zahl der generierten Ladungsträger groß ist, damit ist das Brechungsindexinkrement Δn dem Betrage nach ebenfalls groß, besitzt aber ein negatives Vorzeichen. Die bei einer Bestrahlung mit einem Laserstrahl, der eine gaußförmige Intensitätsverteilung aufweist, entstehende Brechungsindexverteilung im Halbleitermaterial bewirkt, daß die optische Weglänge d (n0-Δn) in den Randbereichen der Verteilung, den Bereichen mit geringer Intensität, groß ist, da der Betrag |Δn| klein ist. Die optische Weglänge in dem mittleren Bereich der Verteilung, dem Bereich mit hoher Intensität, ist dagegen klein, da der Betrag |Δn| groß ist. Für die Laserstrahlung bildet sich eine Zerstreuungslinse im Halbleitermaterial der refraktionsgesteuerten Schicht 7 heraus. Die Brennweite dieser Zerstreuungslinse ist von der Form des Intensitätsprofils, von der absoluten Intensität der Laserstrahlung und der Größe des Fokusflecks, der im Halbleitermaterial erzeugt wird, abhängig.With knowledge and targeted use of the aforementioned physical relationships, the refraction-controlled is in the semiconductor material Layer 7 generates a refractive index profile that reproduces itself spatially and temporally in accordance with the intensity distribution of the incident laser radiation. The charge carriers generated in the semiconductor material of the refraction-controlled layer 7 by the striking laser radiation and the electronic Kerr effect make a contribution to the refractive index with a negative sign, i.e. the local intensity curve of the radiation corresponds to a refractive index curve in the semiconductor material of the refraction-controlled layer 7, with the points being of high intensity The number of charge carriers generated is large, so the refractive index increment Δn is also large in amount, but has a negative sign. The refractive index distribution in the semiconductor material which occurs when irradiated with a laser beam which has a Gaussian intensity distribution has the effect that the optical path length d (n 0 -Δn) is large in the edge regions of the distribution, the regions with low intensity, since the amount | Δn | is small. In contrast, the optical path length in the middle region of the distribution, the region with high intensity, is small because the amount | Δn | is great. A diverging lens for the laser radiation is formed in the semiconductor material of the refraction-controlled layer 7. The focal length of this diverging lens depends on the shape of the intensity profile, the absolute intensity of the laser radiation and the size of the focal spot that is generated in the semiconductor material.
Das Halbleitermaterial der refraktionsgesteuerten Schicht 7 ist so ausgewählt, daß für Laserstrahlung kleiner Intensität eine geringe Absorption auftritt. Die Erzeugung von Ladungsträgern erfolgt über eine 2- Photonenabsorption. Der elektronische Kerr-Effekt ist eine Eigenschaft des Materials unter Strahlungseinfluß.The semiconductor material of the refraction-controlled layer 7 is selected so that low absorption occurs for low-intensity laser radiation. Charge carriers are generated via 2-photon absorption. The electronic Kerr effect is a property of the material under the influence of radiation.
Anhand der Fig. 2 wird der Einfluß der durch die intensitätsabhängige Brechungsindexverteilung im Halbleitermaterial der refraktionsgesteuerten Schicht 7 erzeugten Zerstreuungslinse auf die Strahlungsfeldverteilung (Durchmesser) im Resonator beschrieben. In der Fig. 2 ist lediglich der Strahlenverlauf vom nichtlinearen Reflektor 2 in Richtung zur Blende 4 dargestellt. Nach dem Durchgang durch das Halbleitermaterial der refraktionsgesteuerten Schicht 7, der Reflexion an der hochreflektierenden Schicht 8 und erneutem Durchgang durch das Halbleitermaterial der refraktionsgesteuerten Schicht 7, hat die örtliche Intensitätsverteilung für Bereiche kleiner Intensitäten im zeitlichen Impulsverlauf einen größeren Durchmesser 12 als die örtliche Intensitätsverteilung 11 , die den Bereichen des Impulses mit hoher Intensität (Impulsspitze) entspricht. Dadurch wird eine Verlustmodulation realisiert, die mit zunehmender Intensität eine positive Rückkoppelcharakteristik (Verlustverringerung) besitzt.2, the influence of the refraction-controlled by the intensity-dependent refractive index distribution in the semiconductor material Layer 7 produced a diverging lens on the radiation field distribution (diameter) described in the resonator. 2 only shows the beam path from the non-linear reflector 2 in the direction of the diaphragm 4. After passing through the semiconductor material of the refraction-controlled layer 7, reflecting on the highly reflective layer 8 and again passing through the semiconductor material of the refraction-controlled layer 7, the local intensity distribution for areas of low intensities in the temporal pulse profile has a larger diameter 12 than the local intensity distribution 11. which corresponds to the areas of the pulse with high intensity (pulse peak). As a result, loss modulation is realized, which has a positive feedback characteristic (loss reduction) with increasing intensity.
Der Abstand a der Linse 6 zum nichtlinearen Reflektor 2 ist derart gewählt, daß die Laserstrahlung mit hoher Intensität in der reflektierten Richtung, vom nichtlinearen Reflektor 2 in Richtung zum ersten Reflektor 1 , einen Strahlengang 11 aufweist, der an der Blende 4 einen geringeren Durchmesser aufweist als der Blendendurchmesser, so daß diese reflektierten Strahlen die Blende ungehindert passieren. Für die Laserstrahlung mit geringer Intensität wird im Linsensystem ein Strahlengang 12 erzeugt, der an der Blende 4 einen größeren Durchmesser aufweist als der Blendendurchmesser, so daß diese reflektierten Strahlen die Blende nicht passieren können.The distance a of the lens 6 to the non-linear reflector 2 is selected such that the laser radiation with a high intensity in the reflected direction, from the non-linear reflector 2 in the direction of the first reflector 1, has a beam path 11 which has a smaller diameter at the diaphragm 4 than the aperture diameter, so that these reflected rays pass the aperture unhindered. For the low-intensity laser radiation, a beam path 12 is generated in the lens system, which has a larger diameter at the diaphragm 4 than the diaphragm diameter, so that these reflected rays cannot pass through the diaphragm.
Der Abstand a der Linse 6 zum Reflektor 2 mit der refraktionsgesteuerten Schicht 7, wird durch die Nichtlinearität der refraktionsgesteuerten Schicht 7, durch die Ladungsträgerrelaxationszeit sowie durch die pumpleistungsabhängige Laserleistung der Laserquelle 3 bestimmt. Aus diesen Größen läßt sich nach den vorgenannten Beziehungen die Stärke der durch die Laserstrahlung in der refraktionsgesteuerten Schicht 7 erzeugten Zerstreuungslinse ermitteln. Unter Berücksichtigung dieser Größe kann unter Anwendung bekannter Bemessungsregeln für den Resonator der Abstand a zwischen der Linse 6 und dem Reflektor 2 berechnet werden.The distance a between the lens 6 and the reflector 2 with the refraction-controlled layer 7 is determined by the non-linearity of the refraction-controlled layer 7, by the charge carrier relaxation time and by the laser laser 3, which is dependent on the pump power, and the pump power. The magnitude of the diverging lens produced by the laser radiation in the refraction-controlled layer 7 can be determined from these variables in accordance with the aforementioned relationships. Taking this size into account the distance a between the lens 6 and the reflector 2 can be calculated using known dimensioning rules for the resonator.
In der Fig. 3 ist die normierte Transmissionsänderung an der Blende 4 in Abhängigkeit von der Laserintensität für unterschiedliche Abstände a der Linse 6 zur refraktionsgesteuerten Schicht 7 dargestellt.3 shows the normalized transmission change at the aperture 4 as a function of the laser intensity for different distances a from the lens 6 to the refraction-controlled layer 7.
Die durchgezogene Kurve stellt einen Ausschnitt aus dem Impulsverlauf, nämlich den rechten Halbimpuls der durch die Laserquelle 3 erzeugten (Laserstrahl) dar, wobei die Intensität des Laserstrahls auf der rechten vertikalen Achse aufgetragen ist. Die Kurven A, B und C verdeutlichen die während des impulsverlaufs auftretende Transmissionsänderung an der Blende 4 bzw. dem Faserkern einer faserförmigen Laserquelle in Abhängigkeit vom Abstand a zwischen der Linse 6 und der refraktionsgesteuerten Schicht 7, wobei die Transmissionsänderung auf der linken vertikalen Achse aufgetragen ist. Der Abstand a zwischen der Linse 6 und der refraktionsgesteuerten Schicht 7 ist kleiner als die Linsenbrennweite der Linse 6 und beträgt: für die Kurve A; a = f - 8,4 μm für die Kurve B a = f - 9,2 μm für die Kurve C a = f - 7,6 μm.The solid curve represents a section of the pulse shape, namely the right half pulse of the laser source 3 generated (laser beam), the intensity of the laser beam being plotted on the right vertical axis. Curves A, B and C illustrate the change in transmission occurring at the aperture 4 or the fiber core of a fibrous laser source as a function of the distance a between the lens 6 and the refraction-controlled layer 7, the change in transmission being plotted on the left vertical axis . The distance a between the lens 6 and the refraction-controlled layer 7 is smaller than the lens focal length of the lens 6 and is: for the curve A; a = f - 8.4 μm for curve B a = f - 9.2 μm for curve C a = f - 7.6 μm.
Die Kombination der Linse 6 und die durch die im Laserresonator zirkulierende Intensität erzeugte Zerstreuungslinse muß so ausgelegt sein, daß für kleine Intensitäten eine schlechtere Transmission durch die Blende bzw. den Faserkern im Vergleich zu hohen Intensitäten realisiert wird. Damit wird eine intensitätsabhängige (nichtlineare) Verlustmodulation realisiert, die zur Kopplung der Moden des Lasers und damit zur Erzeugung kurzer Impulse führt. Anhand der Fig. 3 ist deutlich erkennbar, daß die Transmission des Laserstrahls an der Blende 4 mit abnehmender Intensität stark abnimmt. Aufgrund der Umläufe des Impulses im Resonator gehen innerhalb kürzester Zeiträume die Strahlenanteile mit geringerer Intensität an der Blende 4 verloren, so daß nur Strahlenanteile mit hoher Intensität im Resonator verbleiben und somit ein kurzer Impuls mit hoher Intensität erzeugt wird. The combination of the lens 6 and the diverging lens generated by the intensity circulating in the laser resonator must be designed in such a way that poorer transmission through the aperture or the fiber core is realized for small intensities compared to high intensities. In this way, an intensity-dependent (non-linear) loss modulation is realized, which leads to the coupling of the modes of the laser and thus to the generation of short pulses. 3 that the transmission of the laser beam at the aperture 4 decreases sharply with decreasing intensity. Due to the circulation of the pulse in the resonator, the radiation components with lower intensity go within a very short period of time lost at the aperture 4, so that only radiation components with high intensity remain in the resonator and thus a short pulse with high intensity is generated.

Claims

Patentansprüche claims
1. Vorrichtung zur Erzeugung kurzer Laserimpulse durch passive Modenkopplung mit einem Resonator, der zwei Reflektoren (1 , 2) und ein dazwischen angeordnetes laseraktives Medium (3) aufweist, wobei ein Reflektor (2) optisch nichtlineare Eigenschaften hat und aus einer hochreflektierenden Schicht (8) und einer auf der dem laseraktives Medium (3) zugewandten Seite der hochreflektierenden Schicht (8) angeordneten Halbleiterschicht (7) besteht, die einen intensitätsabhängigen Brechungsindex aufweist, wobei für das Halbleitermaterial der Halbleiterschicht (7) folgende Bedingungen gelten:1.Device for generating short laser pulses by passive mode coupling with a resonator, which has two reflectors (1, 2) and a laser-active medium (3) arranged between them, one reflector (2) having optically non-linear properties and consisting of a highly reflective layer (8 ) and a semiconductor layer (7) arranged on the side of the highly reflective layer (8) facing the laser-active medium (3), which has an intensity-dependent refractive index, the following conditions applying to the semiconductor material of the semiconductor layer (7):
- durch 2-Photonenabsorption findet eine strahlungsinduzierte Ladungsträgergeneration statt, die zu einer Brechungsindexänderung führt, - im Bereich großer Intensität kommt es zu einer Verringerung des Brechungsindex, im Bereich kleiner Intensität kommt es zu einer Vergrößerung des Brechungsindex,- 2-photon absorption results in a radiation-induced charge carrier generation that leads to a change in the refractive index, - in the area of high intensity there is a reduction in the refractive index, in the area of low intensity there is an increase in the refractive index,
- für die Energie des Laserphotons hv bezogen auf den Bandabstand des Halbleitermaterials Eg gilt: hv < Eg < 1 ,4hv, - die Ladungsträgerrelaxationszeit in dem Halbleitermaterial ist wesentlich kürzer als die Pulsumlaufzeit im Resonator und größer oder gleich der Dauer der erzeugten Impulse, d a d u r c h g e k e n n z e i c h n e t , daß im Strahlenverlauf der vom optisch nichtlinearen Reflektor (2) reflektierten Laserstrahlung Mittel zur Begrenzung des Durchmessers der Laserstrahlung angeordnet sind und auf der dem optisch nichtlinearen Reflektor (2) zugewandten Seite der genannten Mittel ein aus mindestens zwei Linsen (5, 6) bestehendes Linsensystem angeordnet ist, wobei die erste auf die genannten Mittel folgende Linse (5) eine kollimierende Wirkung und die darauffolgende zweite Linse (6) eine fokussierende Wirkung auf die Laserstrahlung hat, und der Abstand (a) zwischen der zweiten Linse (6) und der Halbleiterschicht (7) des optisch nichtlinearen Reflektors (2) derart eingestellt ist, daß der sich am Ort der genannten Mittel einstellende Durchmesser der Intensitätsverteilung für kleine Intensitäten (12) größer ist als der Durchmesser des Mittels zur Begrenzung des Durchmessers der Laserstrahlung.- for the energy of the laser photon hv in relation to the bandgap of the semiconductor material E g : hv <E g <1, 4hv, - the charge carrier relaxation time in the semiconductor material is considerably shorter than the pulse circulation time in the resonator and greater than or equal to the duration of the pulses generated, characterized in that means for limiting the diameter of the laser radiation are arranged in the beam path of the laser radiation reflected by the optically non-linear reflector (2) and on the side of said means facing the optically non-linear reflector (2) there is at least two lenses (5, 6) Lens system is arranged, wherein the first lens (5) following the said means has a collimating effect and the subsequent second lens (6) has a focusing effect on the laser radiation, and the distance (a) between the second lens (6) and the Semiconductor layer (7) of the optically non-linear reflector (2) in this way it is set that the diameter of the intensity distribution for small intensities (12) which occurs at the location of the said means is larger than the diameter of the means for limiting the diameter of the laser radiation.
2. Vorrichtung nach Anspruch 1 , dadurch gekennzeichnet, daß die Halbleiterschicht (7) eine Dicke besitzt, die größer oder gleich der verwendeten Wellenlänge der Laserstrahlung ist.2. Device according to claim 1, characterized in that the semiconductor layer (7) has a thickness which is greater than or equal to the wavelength of the laser radiation used.
3. Vorrichtung nach Anspruch 1 , dadurch gekennzeichnet, daß die dem laseraktiven Medium (3) zugewandte Seite der Halbleiterschicht (7) entspiegelt ist.3. Device according to claim 1, characterized in that the side of the semiconductor layer (7) facing the laser-active medium (3) is non-reflective.
4. Vorrichtung nach Anspruch 1 dadurch gekennzeichnet, daß die Halbleiterschicht (7) aus ionenimplantiertem GaAs oder aus LT-GaAs oder aus amorphem Si besteht.4. The device according to claim 1, characterized in that the semiconductor layer (7) consists of ion-implanted GaAs or LT-GaAs or amorphous Si.
5. Vorrichtung nach nach Anspruch 1 , dadurch gekennzeichnet, daß bei der Verwendung von stab- oder plattenförmigen laseraktiven Medien (3) das Mittel zur Begrenzung des Durchmessers der Laserstrahlung eine Blende (4) ist, die zwischen dem nichtlinearen Reflektor (2) und dem laseraktiven Medium (3) angeordnet ist.5. The device according to claim 1, characterized in that when using rod-shaped or plate-shaped laser-active media (3) the means for limiting the diameter of the laser radiation is an aperture (4) between the non-linear reflector (2) and the laser-active medium (3) is arranged.
6. Vorrichtung nach Anspruch 1 , dadurch gekennzeichnet, daß bei der Verwendung von faserförmigen laseraktiven Medien (3) die Eintrittsfläche des Faserkerns das Mittel zur Begrenzung des Durchmessers der Laserstrahlung ist. 6. The device according to claim 1, characterized in that when using fibrous laser-active media (3), the entrance surface of the fiber core is the means for limiting the diameter of the laser radiation.
7. Vorrichtung nach Anspruch 1 , dadurch gekennzeichnet, daß der Reflektor (2) mit der hochreflektierenden Schicht (8) auf einem Träger (9) angeordnet ist.7. The device according to claim 1, characterized in that the reflector (2) with the highly reflective layer (8) is arranged on a carrier (9).
8. Vorrichtung nach Anspruch 1 , dadurch gekennzeichnet, daß durch geeignete Dimensionierung der Schichtdicke der Halbleiterschicht (7) sowie der Wege in den optischen Medien (3, 5, 6) der Grad der Kompensation der refraktiven Effekte gezielt einstellbar ist, da die Brechungsindexinkremente für das refraktionsgesteuerte Halbleitermaterial (7) und die optischen Medien (3, 5, 6) umgekehrte Vorzeichen aufweisen. 8. The device according to claim 1, characterized in that by appropriate dimensioning of the layer thickness of the semiconductor layer (7) and the paths in the optical media (3, 5, 6), the degree of compensation of the refractive effects is selectively adjustable, since the refractive index increments for the refraction-controlled semiconductor material (7) and the optical media (3, 5, 6) have opposite signs.
EP00930991A 1999-03-25 2000-03-24 Device for producing short pulses by passive mode lock Expired - Lifetime EP1166403B1 (en)

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DE19954109A DE19954109C2 (en) 1999-03-25 1999-11-02 Device for generating short laser pulses with passive mode coupling through 2-photon absorption
PCT/DE2000/000939 WO2000059082A1 (en) 1999-03-25 2000-03-24 Device for producing short pulses by passive mode lock

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